1. Field of the Invention
The subject invention relates to video display apparatus and, in particular, to suppressing moire disturbances in the displaying of video signals on the video display apparatus.
2. Description of the Related Art
Moire is a term commonly used to describe disturbances on a video display which look like waves on water. A common example in the real world is the interference between two fences. Moire appears when two sampling processes do not match each other, and there is insufficient filtering.
The description of moire on video monitors is complex, because of the two dimensional nature and the completely different sampling structure of the screen of the monitor and that of the video signal. An easy way to describe the origins is given in the one-dimensional time and frequency domain. FIGS. 1A-1C show the structure of a sampling process of an incoming continuous signal. As shown in FIG. 1A, the sampling process may be done by an ideal analog-to-digital converter. The output data s(n) (FIG. 1C) are time discrete samples of the input signal s(t) (FIG. 1B) with a spacing of 1/f.sub.s, where f.sub.s is the sampling frequency. The spectrum of the input signal is repeated by the sampling frequency and all multiples of it. These frequencies will be called carriers, because their behavior is comparable to an amplitude modulation.
Moire disturbances are caused by alias frequencies and beat frequencies. Alias frequencies appear when the repetition spectrum of the first carrier overlaps with the baseband. In this case, high signal frequencies cause low repetition frequencies. Disturbances caused by aliasing are not removable without loss of signal information, when the alias frequency occurs within the baseband bandwidth. The usable baseband is limited by the Nyquist frequency, which is half the sampling frequency.
Beat frequencies near the Nyquist limit are shown in FIGS. 2A-2C. The input signal (FIG. 2A) is below the Nyquist limit and the sampling process gives an additional frequency line at f.sub.s -f.sub.o =0.53f.sub.s. The sampled signal (FIG. 2B) has a period of 1/f.sub.o =1/(0.47fs), which is the signal frequency. Due to the fact that the signal frequency and the repetition frequency are close together, as shown in FIG. 2C, the frequency difference f.sub.b =.linevert split.f.sub.o -(f.sub.s -f.sub.o).linevert split.=.linevert split.f.sub.s -2f.sub.o.linevert split. can be seen as a beat frequency causing modulation in the sampled signal. The beat frequency is just the frequency difference of two physical frequencies, therefore, the beat frequency itself is not a physical frequency. To remove a beat frequency, at least the higher physical frequency must be suppressed. Unfortunately, beat frequencies often come along with alias frequencies.
FIGS. 3A-3C show a time discrete (or digital) low-pass filter along with the input signal spectrum .linevert split.Si(f).linevert split. and the output signal spectrum .linevert split.So(f).linevert split.. An important property of digital filters is the symmetrical frequency response to the Nyquist frequency. Low frequency aliasing cannot be suppressed without significant loss of signal information. In some cases, when the cutoff frequency is below the Nyquist limit, high frequency aliasing can be suppressed along with disturbing beat frequencies. Unfortunately, high signal frequencies would be suppressed in the same way. Beat frequencies can be removed by a digital low-pass filter when the cutoff frequency is significantly below the Nyquist limit.
The two-dimensional sampling process of the video signal creates carrier amplitudes at multiples of the video format, or so-called "resolution". For instance, FIG. 4 shows the frequency space of a graphics standard in the format 1600.times.1200 pixels (N.sub.x.multidot.N.sub.y) The first carrier frequency is determined by the distance of two pixels. Therefore, the first carrier amplitudes become 1/N.sub.x and 1/N.sub.y, which is 1600 cy/pw and 1200 cy/ph (1600 cycles per picture width, and 1200 cycles per picture height).
As shown in FIG. 4, the Nyquist limit has a rectangular shape with borders at half of the sampling frequency. The video format 1600.times.100 pixels has a maximum resolution of 800 cy/pw in the horizontal, and 600 cy/ph in the vertical directions. The three patterns describe the limits at the Nyquist limit. In the horizontal direction, the Nyquist limit is given by alternating pixel amplitudes, while in the vertical, the Nyquist limit is given by alternating line amplitudes, and in the diagonal direction, by alternating pixel and line amplitudes, resulting in a checkerboard pattern.
Video moire appears mostly at the Nyquist limit. The two patterns, checkerboard and alternating pixels (see FIG. 4), are most critical. Both have, in common, the horizontal Nyquist limit. Therefore, it is sufficient to suppress only the area around the horizontal Nyquist limit. In most cases, a two-dimensional low-pass filter is not needed, a one-dimensional horizontal low-pass filter is sufficient.
Linear low-pass filters have several disadvantages. The resolution will decrease and high contrast transitions will be muted resulting in the picture appearing less sharp. Also single lines or fine details will be seriously suppressed, and the picture impression becomes weak. Well-known non-linear filters, e.g., median filters, can preserve sharp edges, but small details will still be suppressed. Additionally, these filters create a certain amount of alias disturbances.